1. Technical Field
The invention generally relates to pulse width modulation (PWM) amplifiers and, more particularly, to an interleaved pulse width modulation amplifier having a time offset that may be incorporated or removed, or increased or decreased.
2. Related Art
Pulse width modulation (PWM) amplification for audio applications has been used to increase efficiency by incorporating output devices that act as switches. In PWM amplifiers, an audio input signal is represented by pulse width modulated waveform. Specifically, an inputted audio signal modulates the width of an ultrasonic rectangular waveform. The modulated waveform is then low-pass filtered, and the resultant analog signal is used to drive the load or loudspeaker. As opposed to linear mode amplification, the transistors that amplify the signal operate in saturation mode being either fully on or fully off. The output transistors are aligned in half-bridge pairs such that one produces a more positive voltage, while the other produces a more negative voltage.
The most common form of pulse width modulated amplifiers, known as class-D amplifiers, are theoretically 100% efficient because the output transistors are either completely on, or completely off. These amplifiers, however, are problematic because the switching of the transistors must be very precisely controlled.
In a class-D amplifier, the switches operate in time alternation. Ideally, the switches are timed perfectly such that one transistor instantaneously turns off, while the other instantaneously turns on. Realistically, however, there may be a delay before a transistor provides an output. The time between the conduction intervals of the two switches when neither switch is on is known as deadtime. Deadtime results in a loss of control that produces distortion and therefore should be minimized. Conversely, with insufficient deadtime, a timing error could cause both the positive and negative switching transistors to be on at the same time. While the time of the overlap may be small, it creates a high shoot-through current that could destroy the output transistors.
To address this issue, Crown Audio developed the opposed current converter, the features of which are discussed in U.S. Pat. No. 5,657,219. In the opposed current converter, alternatively designated as a BCA® (Balanced Current Amplifier) or class-I amplifier, the positive and negative switching pulses are interleaved instead of alternating. Where the audio input signal is at a zero-crossing, i.e. where no signal is to be outputted, the switches turn on and off simultaneously at a 50% duty cycle. As a result the positive and negative signals cancel each other out, a null output signal is provided. Where the incoming signal is going positive, the duty cycle of the positive switch increases, and the duty cycle of the negative switch decreases. Where the incoming signal is going negative, the converse occurs.
One reason why the class-I amplifier is advantageous is that the pulses are centered on each other, and not on when one signal turns off. The class-I amplifier also requires less low-pass filtering to eliminate the switching signal from the output. Further discussion of the class-I amplifier may be found in the U.S. Pat. No. 5,657,219, the inventor's white paper, “Reinventing the Power Amplifier—BCA,” and Crown Audio's Understanding Class-I primer, which are incorporated by reference.
When constructing interleaved PWM amplifiers, circuits are designed to reduce the amount that the switches crosstalk to avoid the scenario in which the decision process of one would be based upon the results of another. The problem of crosstalk is enhanced where signal levels are small. This is because even a minor timing error represents a relatively large distortion compared to the ideal response. The potential for crosstalk distortion is most acute with integrated circuit PWM amplifiers in which the multiple modulators and output stages all share a common substrate and package.
To address crosstalk problems, one may time delay the main switching signal for some of the various signal paths of an interleaved amplifier such that fewer of the small signal modulation event edges coincide in time. This technique is shown for an interleave of two full-bridge amplifier in U.S. Pat. No. 6,373,336 by Anderskouv & Risbo. As shown in FIG. 1, such a system incorporates a time delay element 140 in one of the PWM branches. This system claims to addresses the problem of inadvertent zero-crossing distortion due to cross-talk by introducing a time delay that effectively moves the switching noise of one modulator and power stage to a non-zero signal portion of the modulation cycle of the second modulator and power stage. Alternatively, a modulating waveform may be delayed in one pulse width modulator branch, thereby creating a time offset with respect to the other pulse width modulator branch. The introduction of hysteresis in the modulator can also be used to create delay.
FIG. 1 is an example of a simple interleaved PWM amplifier modulator 100 that receives an input signal from a signal source 110. The amplifier modulator 100 splits the input into two branches. The first branch includes an inverting block 120 The inverting block 120 is connected with a first pulse width modulator 130, referenced as PWM A. PWM A 130 is connected with a first half-bridge 150, referenced as half bridge A. The output of half-bridge A 150 is then connected with the load 160.
The second branch includes a non-inverting block 125 that is connected with a second pulse width modulator 135, referenced as PWM B. PWM B 135 is connected with a delay unit 140. The delay unit 140 is connected to a second half-bridge 155, referenced as half-bridge B. Half-bridge B 155 is then connected with the load 160.
In this system, the cross-talk noise that may otherwise result in distortions that are audible when near input signal zero-crossing conditions are “pushed” away from the zero-crossing by the introduction of a delay by the delay unit 140. The result is an error that can be more easily audibly masked by the now larger signal that is required to bring the noise making output into time coincidence with the other modulator's moment of switching. The introduction of delay approach, however, has limitations that make it inappropriate to use in certain conditions. A primary problem with this approach is that when the time offset is added, feedback signals are corrupted. Because of the addition of the time delay, pulse width modulation signals are also affected. Thus, PWM spectra that were formerly suppressed, may appear as distortion in the audible frequency spectrum.